mach: switch to uv for managing python venv (#34504)

This patch switches servo to use `uv` for both installing a pinned
Python version as well as installing the dependency packages using
`uv`'s pip compatible interface. It also introduces a new 'composite'
GitHub action to setup python in the different CI workflows.

There is no support for externally managed python installations and
virtual environments. These could be added in the future.

Fixes #34095

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>

.github/workflows/linux.yml

@@ -139,13 +139,10 @@ jobs:
       - name: Set LIBCLANG_PATH env # needed for bindgen in mozangle
         if: ${{ ! fromJSON(needs.runner-select.outputs.is-self-hosted) && !inputs.upload }} # not needed on ubuntu 20.04 used for nightly
         run: echo "LIBCLANG_PATH=/usr/lib/llvm-14/lib" >> $GITHUB_ENV
-      - uses: actions/setup-python@v5
-        if: ${{ ! fromJSON(needs.runner-select.outputs.is-self-hosted) }}
+      - name: Setup Python
+        uses: ./.github/actions/setup-python
         with:
-          python-version: '3.10'
-      - name: Bootstrap Python
-        if: ${{ ! fromJSON(needs.runner-select.outputs.is-self-hosted) }}
-        run: python3 -m pip install --upgrade pip
+          skip-python-setup: ${{ fromJSON(needs.runner-select.outputs.is-self-hosted) }}
       - name: Bootstrap dependencies
         if: ${{ ! fromJSON(needs.runner-select.outputs.is-self-hosted) }}
         run: |